The present invention relates to an optical device suitably adapted for an image reading optical system such as a facsimile system, and to a document reader using such an optical device.
One of major technical problems to be solved in scanning and photoelectrically converting an image using a one- or two-dimensional solid-state image sensor such as a CCD image sensor is suppression of the moire of a scanned image.
FIGS. 1A and 1B show waveforms for explaining how a moire is generated. FIG. 1A shows the relationship between openings 2, 3, 4, 5, 6 etc. on an imaging surface 1 of a solid-state image sensor corresponding to picture elements, and the waveform of a sinusoidal pattern of the incident light on the imaging surface 1 and having a pitch l.sub.1 .apprxeq.2l.sub.2 (where l.sub.2 is a black interval). In this case, if the opening sensitivity distribution is assumed to be uniform, the photoelectric conversion output of each picture element of the solid-state image sensor has a moire in which a very strong contrast of three times or more between a maximum value V.sub.3 and a minimum value V.sub.l is generated, as shown in Fig. 1B. Note that reference symbol d denotes a picture-element pitch.
In order to suppress such a moire, it is effective to form on the imaging surface a double or multiple image whose components are shifted along the scanning direction.
FIGS. 2A and 2B show waveforms demonstrating the moire suppression effect obtained when a double image is formed and are similar to FIGS. 1A and 1B. Referring to FIG. 2A, the solid line and the dotted line respectively show waveforms of a sinusoidal pattern of the incident light which is shifted by a displacement S corresponding to half the picture element pitch d in the scanning direction. FIG. 2B shows the photoelectric conversion output of each picture element of the solid-state image sensor when such a double image is formed on the imaging surface 1. Thus, the moire contrast is reduced. The magnitude of the displacement of the double image must be determined in accordance with an image pattern to be processed, a desired resolution, or the like.
In order to form such a double image, a method is adopted wherein a quartz plate is inserted in front of the projection plane of the optical system, i.e., the imaging surface, and a double image having components with a predetermined displacement is formed utilizing the birefringence of the quartz plate.
In another method, a number of rod lenses of focusing light guides are arranged along the scanning direction to form rod lens rows. Light rays reflected from a document or the like are divided by these two rod lens arrays so as to form a double image having components with a predetermined displacement on an imaging surface of a sensor.
In the former method, when a contact-type image sensor (a relatively large image sensor) is used, in order to form on the imaging surface of the contact-image sensor an erect image of equal size to the original, a large quartz plate is required and the cost of the optical system is increased. In the latter method, the displacement of the two image components must be controlled precisely. For this reason, stability in precision is poor, and the device becomes expensive.
Various proposals have recently been made to provide a color image sensor. FIG. 3 shows a plan view showing a typical arrangement of color separation filters mounted on the imaging surface of a color image reading line image sensor. A number of openings are formed on the imaging surface at a rate of three per picture element and at a pitch of p/3. Red, green and blue transmission filters R, G and B are arranged on the respective openings in the order shown in FIG. 3. An image is separated into three colors by these filters and is read.
FIG. 4 is plan view showing another arrangement of tricolor separation filters mounted on the imaging surface of a line image sensor. In this case, a number of openings are formed at a rate of two per picture element, and the respective color transmission filters are arranged at a pitch of p/2 in the order of R, G, B, G, R, G, B, G, and so on.
In such a line image sensor, when a black-and-white pattern is imaged using an imaging means such as a focusing rod lens array in an imaging surface, a color misregistration is caused in addition to a moire, and the image quality is significantly degraded.
FIGS. 5A to 5C are illustrations for explaining misregistration. Assume a case wherein an erect image equal in size to a document consisting of a white region (denoted by W) and a black region (hatched region) connected in the horizontal scanning direction is formed on the imaging surface of a line image sensor shown in FIG. 5B. An image reproduced in accordance with a reading output from the line image sensor is shown in FIG. 5C. Referring to FIG. 5C, a blue region (denoted by B) of one picture-element width is formed at the boundary between a white region (denoted by W) and a black region (hatched region). This is caused by the following. Of openings 7, 8 and 9 of the line image sensor corresponding to the boundary between the black and white regions of the document, only the opening 9 receives light (white light). Thus, this portion produces an output which is the same as that obtained from a blue image.
Although the above description is made with reference to the case of a line image sensor having the filter arrangement shown in FIGS. 1A and 1B, the same applies to the case of a line image sensor having the filter arrangement shown in FIGS. 2A and 2B.
In order to prevent color misregistration and the moire in a camera tube such as a television camera, a quartz plate is arranged in front of the imaging surface of an image sensor as described above, and a double image of an incident image is formed by the birefringence of the quartz plate. This can be also performed by the tricolor tube method.
However, in the former method, the resolution characteristics are degraded considerably, and misregistration is still not sufficiently prevented. It is therefore difficult to apply this method to a document reader such as a facsimile system, which must be capable of producing details of an image in the form of a hard copy. In the case of a contact-type document reader where an erect image equal in size to a document is formed on a line image sensor, a very large quartz plate corresponding to a maximum read width is required. However, such a big quartz plate is expensive, thus increasing the overall cost of the device.
In the latter method, since the number of image sensors is increased, this also increases the cost of the device.